This invention relates generally to a radiosurgery system employing multiple beams of radiation focused onto a stereotactically localized target, and more particularly to stereotactic radiosurgery apparatus affording greatly improved mechanical accuracy in the focusing of radiation from a moving linear accelerator with respect to a moving stereotactic frame.
In 1951, Dr. Lars Leksell coined the term "radiosurgery", to describe the concept of focusing multiple beams of external radiation on a stereotactically localized intracranial target. After experimentation with standard X-ray treatment devices, proton beam, and linear accelerators, he and his collaborators developed a device which is called the GAMMA KNIFE (currently marketed by the Electra Corporation, Stockholm, Sweden). The device consists of a hemispheric array, currently containing 201 Cobalt-60 sources. The radiation from each of these sources is collimated and mechanically fixed, with great accuracy, on a focal point at the center of the hemisphere. When a patient has a suitable lesion for treatment (usually an intracranial arteriovenous malformation), it may be precisely localized with another device called a stereotactic frame. Using the stereotactic apparatus, the intracranial target is positioned at the focal point of the GAMMA KNIFE. Since each of the 201 radiation pathways is through a different area of the brain, the amount of radiation to normal brain tissue is minimal. At the focal point, however, a very sizable dose is delivered which can, in certain cases, lead to obliteration of the lesion. This radiosurgical treatment is, in some instances, a much safer treatment option than conventional surgical methods.
Four GAMMA KNIFE devices are currently being used worldwide for stereotactic radiosurgery (Stockholm, Sweden; Buenas Aires, Argentina; Sheffield, England; Pittsburgh, U.S.A.), and have been used to treat approximately 1500 patients. The results of treatment, as well as many technical issues, have been discussed in multiple publications. Several factors, however, have impeded the widespread usage of this device. First, the device costs about $2.2 Million Dollars, U.S. Second, the Nuclear Regulatory Commission has ruled that this device cannot be shipped loaded in the U.S.A. Consequently, loading must be done on site, necessitating the construction of a portable hot cell. Third, the half life of Cobalt-60 is 5.2 years, which requires reloading the machine, at great expense, every 5-10 years. Fourth, the dosimetry system currently marketed with the device is relatively crude, especially when utilized with more modern imaging modalities such as CT scan and MRI scan.
An alternative method for radiosurgery involves irradiation of intracranial targets with particle beams (i.e., proton or helium). In this instance, one does not rely solely on multiple cross-fired beams of radiation. A physical property of particle beams, called the "Bragg-peak effect", allows one to deliver the majority of the energy of a small number of beams (approximately 12) to a precisely predetermined depth. Multiple publications regarding particle irradiation of intracranial lesions (especially pituitary tumors and arteriovenous malformations) have appeared in the literature. The results have not generally been as good as those obtained with the GAMMA KNIFE. This may, however, be solely a consequence of patient selection criteria. Particle beam devices require the availability of a cyclotron. Only a few such high energy physics research facilities exist in the world.
A third current radiosurgical method uses a linear accelerator (LINAC) as the radiation source. As mentioned above, Leksell rejected the LINAC as mechanically inaccurate. More recently, groups from Europe have reported their methods for radiosurgery with LINAC devices. In the U.S., researchers at the Peter Bent Brigham Hospital in Boston have developed a prototype LINAC system using highly sophisticated computer techniques to optimize dosimetry. Thus far, approximately 12 patients have been treated with good results. This LINAC system, however, suffers from certain mechanical inaccuracies which have limited its use. In addition, the computer dosimetry system employed is very time consuming, rendering the treatment program inefficient.
Currently, there is great interest in radiosurgery. Although the GAMMA KNIFE represents the "gold standard", its great expense and requirement for frequent replenishment of radiation sources have discouraged most potential users. The proton beam devices are never likely to be widely available because of the requirement for high-energy particle beam source (cyclotron). The linear accelerator offers an attractive alternative to such devices. However, a major disadvantage of known linear accelerator based systems is their mechanical inaccuracy.
It is desirable to provide stereotactic radiosurgery apparatus employing linear accelerators which overcomes the disadvantages of known systems, and it is to end that the present invention is directed.